1. Field of the Invention
The invention disclosed in this specification relates to a semiconductor device and a method for manufacturing the semiconductor device.
In this specification and the like, a semiconductor device refers to all types of devices which can function by utilizing semiconductor characteristics; an electro-optical device, a light-emitting display device, a semiconductor circuit, and an electronic device are all semiconductor devices.
2. Description of the Related Art
A technique by which transistors are formed using semiconductor thin films formed over a substrate having an insulating surface has been attracting attention. The transistor is applied to a wide range of semiconductor devices such as an integrated circuit (IC) and an image display device (also simply referred to as display device). A silicon-based semiconductor material is widely known as a material for a semiconductor thin film applicable to a transistor. As another material, an oxide semiconductor has been attracting attention.
For example, a technique in which a transistor is manufactured using a zinc oxide or an In—Ga—Zn-based oxide as an oxide semiconductor is disclosed (see Patent Documents 1 and 2).
Meanwhile, it has been pointed out that when hydrogen enters an oxide semiconductor, a donor is generated at a level close to the conduction band (a shallow level), and the oxide semiconductor becomes an n-type oxide semiconductor. Therefore, some measures need to be taken to prevent hydrogen from entering the oxide semiconductor at the time of depositing the oxide semiconductor. Further, the following technique is also disclosed: fluctuation of a threshold voltage is suppressed by reducing the amount of hydrogen contained in not only the oxide semiconductor but also a gate insulating film in contact with the oxide semiconductor (see Patent Document 3).